BTU Calculator
Find the right room air conditioner size, in BTU per hour, for your space.
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For a 12 ft × 15 ft room (180 sq ft) with average sun exposure, and 2 regular occupants, you'll want a room air conditioner of about 6,000 BTU/hr.
Recommended cooling capacity
6,000 BTU/hr
That's small-room territory — bedrooms, offices, and modest living rooms, covered by the most common window-unit sizes.
Units in this class run on a standard 115 V household outlet. Match to the nearest retail size (5,000 / 6,000 / 8,000 BTU), and resist sizing up "to be safe" — an oversized unit short-cycles and leaves the room cold but humid.
Room area (Energy Star table unit): 180 sq ft
Base table capacity before adjustments: 6,000 BTU/hr
How this works
The calculator implements the published Energy Star room air conditioner sizing method in two stages. Stage one is a table lookup: your room's floor area (length × width — entered in feet or metres, converted internally to square feet, the unit the table is banded in) selects a base capacity from the published area-to-BTU table, which runs from 5,000 BTU/hr for rooms of 100–150 sq ft up to 34,000 BTU/hr for 2,000–2,500 sq ft; areas beyond the table clamp to that final band, and this page's edge cases explain why bigger spaces need a different approach entirely. Stage two applies the four published adjustments in order: very sunny rooms get 10% added to the base figure and heavily shaded rooms get 10% subtracted (sun through glass is a load proportional to the room's overall gain, hence a percentage); kitchen use then adds a flat 4,000 BTU/hr (cooking heat doesn't scale with room size, hence a fixed amount); and each regular occupant beyond two adds 600 BTU/hr (a resting person's approximate heat output). The result is a BTU/hr recommendation you match to the nearest available retail size class — room units come in steps (5,000, 6,000, 8,000, 10,000, 12,000...), so the interpretation band under the result explains which direction to round based on the factors the table can't see, like ceiling height and insulation quality. The base table and all four adjustments come verbatim from the Energy Star guidance US EPA / DOE Energy Star, Room Air Conditioner Buying Guidance (sizing table and adjustments).
Worked example
A 12 ft × 15 ft bedroom — 180 sq ft — with ordinary sun exposure, no kitchen, and two occupants: 180 sq ft falls in the Energy Star table's 150–250 sq ft band, which calls for 6,000 BTU/hr. No adjustments apply (average sun means neither ±10%, no kitchen addition, and occupants don't exceed the two-person baseline), so the recommendation is 6,000 BTU/hr — conveniently an exact retail size class for window units.
A second example: a sunny kitchen-diner
A 360 sq ft open kitchen-diner with large west-facing windows: 360 sq ft lands in the 350–400 band, base 9,000 BTU/hr. The very-sunny adjustment adds 10%, bringing it to 9,900, and kitchen use adds a flat 4,000 BTU on top, for 13,900 BTU/hr. Two people cook and eat there, so no occupant addition applies. The practical purchase is a 14,000 BTU unit — and note that without the two adjustments the raw table said 9,000, a full 35% short: the adjustments are not fine-tuning, they are frequently the difference between a unit that keeps up on a July afternoon and one that never does.
Common mistakes
Sizing by the room the unit sits in, not the space it must cool
A window unit in a living room that opens onto a dining area through a wide archway is cooling both spaces whether you intended it or not. Enter the total connected floor area — sizing only the nominal room can undersize by half, and the unit will run flat-out all afternoon without catching up.
Deliberately oversizing 'to cool faster'
It does cool faster — and then short-cycles: the thermostat satisfies before the unit has dehumidified, leaving cold, damp, cave-like air, and the frequent compressor starts consume more energy than longer steady runs. Energy Star's table targets the capacity that runs long enough cycles to control both temperature and humidity; trust it rather than jumping two sizes.
Ignoring the sun adjustment on a west-facing room
Direct afternoon sun through glass is one of the largest heat loads a room can have, and it's exactly what the +10% adjustment covers. A 350 sq ft west-facing room needs about 9,900 BTU rather than the table's base 9,000 — skipping the adjustment lands you a full retail size class low.
Forgetting the kitchen adjustment when the unit serves a kitchen
The +4,000 BTU kitchen addition is large — often 40–60% on top of a small room's base figure — because ranges, ovens, and refrigerators reject substantial heat. A unit that would be right for a 250 sq ft bedroom is two sizes short in a 250 sq ft kitchen-diner.
Counting square feet from a floor plan that includes closets and walls
Listed home or room areas often include closet footprints and interior wall thickness. Measure the actual open floor the unit conditions — on a small room, 20 sq ft of closet is the difference between the 5,000 and 6,000 BTU bands.
Applying this method to a whole house through central ducts
The Energy Star table is for a single room served by a room unit. Whole-house systems face duct losses, multi-orientation solar gain, and envelope leakage that only a Manual J load calculation captures — extrapolating this table across 2,400 sq ft of house produces a number that looks precise and isn't.
Ignoring the electrical circuit the unit needs
Sizing up sometimes crosses an electrical threshold: most units up to about 12,000 BTU run on a standard 115 V outlet, while many larger units require a dedicated 230 V circuit. Check the spec sheet before buying the bigger unit your calculation suggests — the extra capacity is useless if the wall can't power it.
When this calculator's model stops applying
- Rooms over 2,500 sq ft: the published table ends at 2,500 sq ft → 34,000 BTU, and this calculator clamps larger areas to that last band. Spaces beyond it are genuinely multi-unit or central-system territory, not a bigger window unit.
- Rooms under about 100 sq ft: the table starts at the 5,000 BTU class, which is also the smallest room unit commonly sold — a tiny, well-shaded office may be slightly oversized even at 5,000 BTU, which is acceptable at this scale but explains short cycling in very small rooms.
- Vaulted or double-height ceilings: the table assumes standard ceiling height, and capacity requirements scale with air volume and wall/glass area, not just floor area. For a great room with a 16 ft peak, treat the result as a lower bound and size up a band.
- Server closets, home gyms, and rooms with major equipment heat: the method models people, sun, and kitchens — not a rack of electronics or two people on exercise machines. Add the equipment's actual wattage (1 W ≈ 3.41 BTU/hr) on top of the result manually.
- Extreme climates: the underlying table is calibrated for typical US design conditions. In desert climates with 110°F+ design temperatures or high-humidity tropical settings, local utility or manufacturer derating guidance takes precedence over a national table.
- Heating-mode sizing for heat pumps: BTU heating loads follow different math (design temperature difference and envelope losses), so do not reuse this cooling result to size a heater or the heating side of a reversible unit.
- Portable units with single-hose exhaust: their effective delivered capacity is meaningfully below their nameplate BTU rating because the exhaust hose creates negative pressure that draws warm air in. When the calculator's result sits between sizes and the unit is a single-hose portable, choose the larger.
- Multiple small units versus one large: for an L-shaped or partially divided space, two correctly sized smaller units often outperform one large unit placed in one wing — the table sizes total capacity but says nothing about air distribution, which walls and corners dominate.
FAQ
What is a BTU, and why are air conditioners rated in BTU per hour?
A BTU (British thermal unit) is the heat needed to raise one pound of water by one degree Fahrenheit. An air conditioner's rating is BTU per hour — how much heat it can move out of a room in an hour. A 6,000 BTU/hr unit removes 6,000 BTUs of heat every hour it runs at capacity, which is why the right size depends on how fast heat enters the room: its floor area, sun exposure, occupants, and appliances.
Where do this calculator's numbers come from?
Directly from the published Energy Star room air conditioner sizing guidance: a base table mapping floor area in square feet to capacity in BTU/hr (100–150 sq ft → 5,000 BTU up through 2,000–2,500 sq ft → 34,000 BTU), plus its four adjustments — add 10% for a very sunny room, subtract 10% for a heavily shaded one, add 4,000 BTU for kitchen use, and add 600 BTU per person beyond two regular occupants. The calculator applies exactly those published rules, nothing proprietary.
Is a bigger air conditioner always safer?
No — oversizing is a real failure mode, not a margin of safety. An oversized unit cools the air quickly and shuts off before it has run long enough to remove humidity, leaving the room cold but clammy, and the rapid on/off cycling wears the compressor and wastes energy. Energy Star's guidance exists precisely because 'round up a lot to be safe' produces worse comfort than sizing correctly.
What happens if I undersize instead?
An undersized unit runs continuously on hot days without ever reaching the set temperature, which strains the compressor, drives up the electric bill, and still leaves you warm. If your room lands near the top of a table band — say 245 sq ft in the 150–250 band — and it's sunny or heavily used, the adjustments this calculator applies are what nudge you correctly into the next capacity up.
Do ceiling height and insulation matter?
The Energy Star table assumes typical residential conditions, including standard 8-foot ceilings and reasonable insulation. A room with a vaulted ceiling holds more air volume per square foot of floor, and a poorly insulated room above a garage gains heat faster — both argue for treating a mid-band result as the next band up. The table doesn't take ceiling height as an input, so this judgment stays with you.
How do I measure my room for this calculator?
Length times width of the floor, in feet or metres — the toggle converts either way. For an L-shaped room, split it into two rectangles and add the areas before entering an equivalent length and width (or enter the total as one dimension and 1 as the other, in metric mode, if that's easier). For an open-plan space, measure the entire connected area the unit must cool, not just the room it sits in — cool air doesn't stop at a wide archway.
What counts as a 'sunny' or 'shaded' room?
Energy Star's terms are 'very sunny' and 'heavily shaded.' A room with large south- or west-facing windows getting direct afternoon sun is the +10% case; a north-facing room, a basement, or one shaded by trees or adjacent buildings all day is the −10% case. A room with ordinary mixed exposure is the default — apply neither adjustment.
Why does a kitchen get a flat +4,000 BTU rather than a percentage?
Because cooking heat doesn't scale with room size — a range and oven dump roughly the same heat into a small kitchen as a large one. Energy Star therefore specifies a fixed 4,000 BTU addition for units used in kitchens, and this calculator adds it after the percentage-based sun adjustment, matching the published method.
Does this sizing apply to central air, heat pumps, or mini-splits?
It's published for room air conditioners — window units, through-wall units, and portables. Ductless mini-split sizing is broadly similar for a single zone and this gives a sound first estimate, but central systems and whole-home heat pumps need a full Manual J load calculation accounting for the building envelope, ducts, and orientation — a different exercise a contractor performs. Treat this tool as authoritative for room units, indicative for single-zone mini-splits, and not applicable to central systems.
My result is 6,600 BTU but stores sell 6,000 and 8,000. Which do I buy?
Room ACs come in stepped capacities (5,000, 6,000, 8,000, 10,000, 12,000...), so land on the nearest available size — and when your adjusted result sits between steps, the deciding factors are the ones the table can't see: pick the larger if the room has high ceilings, poor insulation, or you run electronics that generate heat; pick the smaller if the room is well-sealed and used lightly. A result just 10% over a step (like 6,600 over 6,000) usually means the smaller unit is fine.
Does the occupant adjustment count pets or just people?
Energy Star's rule is 600 BTU per person beyond two regular occupants — people, because a resting human emits roughly 300–400 BTU/hr of heat and activity raises it. Pets emit heat too but aren't in the published method; a houseful of large dogs is the kind of judgment factor that argues for the next size up when you're between capacities.
Can I enter metric dimensions even though the result is in BTU?
Yes — the unit toggle accepts room dimensions in metres and converts internally to the square-foot bands the Energy Star table is published in. The result stays in BTU/hr either way because that's the unit room air conditioners are sold by in this market; if you're comparing against a unit rated in watts, 1,000 BTU/hr is about 293 W of cooling power.
Related tools
Insulation Calculatora well-insulated room needs less cooling capacity — if your space runs hot because of a thin attic or bare walls, estimate the insulation job too.More HVAC tools (heating BTU and mini-split estimators) are planned but not yet published.